Emitter-coupled logic (ECL) involves a bipolar form of logic, whereby the bipolar transistors are arranged so that they do not operate at saturation. In recent designs, ECL is often used with a positive power supply. Positive ECL designs are typically referred to as positive- or psuedo-ECL (PECL). PECL can be advantageous as clock speeds increase. For example, PECL can be employed within an output driver circuit to produce high speed output signals. The high speed signals are well suited as a clocking source or as a serial link if applied in complementary, or differential form.
Differential binary signals are defined as the difference between two outputs at dissimilar logic values. For example, one output may have a logical value of 1, while the other, differential output, has the opposite logical value of 0. The differential outputs are typically driven by differential drive transistors. The performance of the drive transistors can vary due to temperature and process variations. For example, an increase in operating temperature will decrease the base-to-emitter forward bias voltage (VBE) of the drive transistors. It is generally accepted that for every 1° Celsius (C.) increase in operating temperature, the forward bias voltage VBE decreases approximately 2 millivolts (mV).
FIG. 1 shows a conventional circuit for compensating for temperature variations. The circuit 100 includes two subcircuits 102 and 104. The subcircuit 102 includes differential drive stages 106 and 108, differential output transistors (devices) 110 and 112, and differential outputs 114 and 116. The subcircuit 104 corrects for the temperature variation in the forward bias voltage VBE. The subcircuit 104 includes a sensing (or replicating) device 118 and an operational amplifier 120.
The sensing device 118 is a replicating transistor. The correcting subcircuit 104 operates on the principle that the forward bias voltage VBE for the sensing device 118 will be the same as the forward bias voltage VBE for the differential output transistors 110 and 112. A current source 122 draws a constant current across the replicating transistor 118 at all times. A replicated voltage across the resistor 124 corresponds to the replicated voltage across the transistor 118. The current IVBE is proportional to the voltage VBE. A current source 126 generates a reference current IREF and has a voltage VREF across it. The current IM is non-zero when VBE<VREF. IM is mirrored such that IOUT2 and IOUT3 are proportional to IM, and hence a function of VBE. IOUT2 and IOUT3 are compensating currents applied to the drive stages 106 and 108.
The subcircuit 104 monitors the base currents of the transistors 110 and 112. When these currents exceed the predetermined threshold of the current IREF, the compensating currents IOUT2 and IOUT3 sink current from the drive stages 106 and 108. This effectively increases the output voltages at the outputs 114 and 116.
In an integrated circuit (IC), the transistors 110, 112, and 118 are formed on the same wafer, and theoretically subject to the same temperature and process variations. However, the sensing device 118 is so distant from the differential output transistors 110 and 112 that the sensing device 118 may not exhibit the same forward bias voltage as the two differential output transistors 110 and 112. This may result in some inaccuracy in the compensating current. In addition, the transistor 118 is typically a much smaller device in area than the output transistors 110 and 112, hence there is significant current density (a potential source of error) difference between the correcting device and the output devices.
The common-emitter current gain, or beta, of a transistor, is a measure of the amount of current gained between the collector and the base of the transistor. Beta typically varies between transistors, even if they are manufactured using the same techniques. Therefore, two otherwise identical transistors in two otherwise identical circuits may have a different common-emitter current gain, thereby affecting the characteristics of the circuit.